Due to persistent security threats and the possibility of terrorist activities, there is a need for deploying high speed, high resolution, and more accurate screening devices at places that are most likely targets of such activities. In addition, there exists a requirement for screening of baggage, cargo and other items for explosives and other illicit materials. This requires a screening system which is capable of discriminating between different materials based on one or more unique features of each material such as effective atomic number, chemical structure, physical density, among other variables.
The use of X-ray computed tomography for the screening of baggage has become fairly common in recent times, since the cross-sectional image data that such imaging systems produce is generally of high quality and of reasonable quantitative accuracy. Known tomographic imaging systems tend to be based on a rotating gantry which carries, as a minimum, an X-ray source with a large stationary array of detectors and more commonly an X-ray source and an opposing array of X-ray detectors which rotate together around the object under inspection. The information collected is reconstructed using known algorithms, such as a filtered backprojection algorithm or an iterative algorithm to produce a two-dimensional image.
In more recent systems, the object is able to move continuously through the imaging plane during data acquisition and, through the use of a cone-shaped X-ray beam with a two dimensional array of detectors, a three-dimensional reconstructed image is produced using filtered backprojection or iterative reconstruction methods. In a further scanning embodiment, a stationary gantry system may be envisaged with a complete ring of rapidly switchable X-ray sources and a sensor array comprising one or more rings of X-ray detectors which may be used to form a three-dimensional image as the item under inspection passes through the imaging plane.
Such images, however produced, are capable of being reconstructed into an image that is substantially determined by the mass attenuation coefficient of the material under inspection. The mass attenuation coefficient is determined through the combination or probabilities of X-ray interaction in the object through the photoelectric effect (absorption), Compton effect (inelastic scattering), Rayleigh effect (elastic scattering) and the density of the material. The individual detector thus sees an intensity of radiation at any point in time which is due to both those primary X-rays which have passed through the object unimpeded (i.e. without being absorbed and without being scattered) and those which have arrived at the detector due to one or more scattering interactions.
The mass attenuation coefficient is equal to the linear attenuation coefficient divided by density. The linear attenuation coefficient is a quantity that characterizes how easily a material or medium can be penetrated by a beam of light, sound particles, or other energy or matter. A large attenuation coefficient means that the beam is quickly “attenuated” or weakened as is passes through the medium, and a small attenuation coefficient means that the medium is relatively transparent to the beam. Therefore, the atomic number of the material under inspection plays a dominant role in determining the effective linear attenuation coefficient through its impact on the probability of photoelectric effect interactions while the density of the material plays a significant role in determining the mass attenuation coefficient of the material.
Thus, there is a need for an improved X-ray inspection system and method that detects the presence of predefined materials based on the mass attenuation coefficients of the materials.